The present invention relates to an oscillator circuit, an oscillation frequency adjusting method of the oscillator circuit, a tuner, and an oscillation frequency adjusting method of a local oscillation circuit of the tuner suitable for use in a. digital satellite tuner or the like of a receiving apparatus for receiving a digital satellite broadcasting, for example.
A general digital-satellite-broadcasting receiving apparatus includes a digital satellite antenna, a digital satellites converter, a digital satellite tuner and so on. A digital. satellite broadcasting signal from a broadcasting satellite is received by the digital satellite antenna. The digital satellite converter converts this digital satellite broadcasting signal received by the digital satellite antenna to a low-frequency signal having a frequency, for example, ranging from 950 MHz to 2,150 MHz to amplify the latter, and then supplies a high-frequency reception signal thus obtained to the digital satellite tuner.
The digital satellite tuner selects a desired transponder from the high-frequency reception signal and demodulates analog base-band signals (I, Q). The tuner further carries out an A/D conversion of these I and Q signals and demodulates a digital signal by using a QPSK demodulator, thereby outputting the demodulated digital signal by a unit of 8 bits in parallel.
A digital satellite tuner shown in FIG. 1 has generally been proposed. The digital satellite tuner shown in FIG. 1 will be described. In FIG. 1, reference numeral 1 represents a high-frequency reception signal input terminal supplied with a high-frequency reception signal having a frequency ranging from 950 MHz to 2,150 MHz, for example from the digital satellite tuner. The high-frequency reception signal from the high-frequency reception signal input terminal 1 is supplied to one input terminal of a mixing circuit 5 for converting a frequency through a serial circuit formed by a high-pass filter 1a for removing, for example, an intermediate-frequency signal of 479.5 MHz, a high-frequency amplifier circuit 1b, a voltage-control type variable band-pass filter 2 for removing, for example, an image frequency signal, an adjacent transponder signal and so on to thereby allow passing of a desired signal out of signals having frequencies ranging from 950 MHz to 2,150 MHz, an automatic gain control circuit 3 and a high-frequency amplifier circuit 4.
Reference numeral 6 represents a voltage-control type variable frequency oscillator circuit forming a local oscillator 10. An oscillator at an output side of the voltage-control type variable frequency oscillator circuit 6 is supplied to one input terminal of a phase comparator circuit 7 forming a PLL circuit (phase locked loop circuit).
Reference numeral 8 represents a central processing unit (CPU) for controlling the receiving apparatus. The central processing unit 8 generates a channel selection signal having a frequency corresponding to a desired channel. This channel selection signal having the frequency corresponding to the desired channel is supplied from the central processing unit 8 to the other input terminal of the phase comparator circuit 7.
The phase comparator circuit 7 compares an oscillations signal from the voltage-control type variable frequency oscillator circuit 6 with the channel selection signal having a frequency corresponding to a desired channel to obtain an error signal at the output side. The phase comparator circuit 7 supplies the error signal obtained at the output side thereof to a control terminal of the voltage-control type variable frequency oscillator circuit 6 through a low-pass filter 9 and also supplies a control voltage corresponding to the channel selection signal obtained at the output side of the low-pass filter 9, to the variable band-pass filter 2 having a varicap and so on, whose capacitance value changes in response to the control voltage.
This arrangement allows the band-pass filter 2 to remove signals other than a desired signal, e.g., an image frequency signal, a next to adjacent transponder signal and so on.
The voltage-control type variable frequency oscillator circuit 6 controls a frequency of the oscillation signal in response to the error signal obtained at the output side of the low-pass filter 9. In the voltage-control type variable frequency oscillator circuit 6, reference numeral 6a represents an amplifier circuit unit and reference numeral 6b represents a resonator circuit unit.
In this case, the phase comparator circuit 7, the low-pass filter 9 and voltage-control type variable frequency oscillator circuit 6 form the local oscillator 10 forming the PLL circuit.
A local oscillation signal from the local oscillator 10 is supplied to the other input terminal of the mixing circuit for converting a frequency. The mixing circuit 5 supplies its output signal to an intermediate-frequency output terminal 14 through a serial circuit formed by a surface acoustic wave filter 12 for passing an intermediate-frequency signal and an amplifier circuit 13. The analog base-band signals (I, Q) are demodulated from the intermediate-frequency signal supplied from the intermediate-frequency output terminal 14, and A/D conversion of the I and Q signals are carried out to thereby carry out the QPSK demodulation.
In the prior art shown in FIG. 1, the mixing circuit 5, the amplifier circuit unit 6a of the voltage-control type variable frequency oscillator circuit 6, and the intermediate- frequency amplifier circuit 11 form an integrated circuit 15.
Since the prior art shown in FIG. 1 employs an upper side heterodyne, if a frequency of the high-frequency reception signal supplied to the one input terminal of the mixing circuit 5 is A, e.g., 950 MHz to 2,150 MHz and a frequency of the intermediate-frequency signal is C, e.g., 479.5 MHz, then a frequency B of the oscillation signal of the local oscillator 10 is EQU B=A+C.
The frequency B of the oscillation signal of the local oscillator 10 ranges from 1,429 MHz to 2,629.5 MHz, and this range is a very high and wide band.
Since the oscillation signal of the local oscillator 10 of the above digital satellite tuner has a very high frequency and has a very wide band, there are such disadvantages that dispersion in characteristics of an oscillator circuit element, a resonator circuit and so on lead to dispersion in frequencies of the oscillation signal of the local oscillator 10.